The present invention relates to the technical field of electric machines. It relates, in particular, to the configuration of a stator bore in axial end parts of a stator, in the region of rotor retaining rings.
In electric machines, such as generators or electric motors, the air gap dimension between an active part of the rotor and a laminated core of the stator constitutes an important parameter for setting the machine characteristics. On the other hand, this dimension also influences the mechanical handleability of the rotors which often weigh several tons. It is therefore obvious that a minimum radial dimension of the air gap is necessary in order to move the rotor of, for example, a generator for a power output of a few 100 MVA into the stator and out of the stator. The axial end faces of such a machine constitute a critical region in this context.
The rotors are normally provided at their ends with rotor retaining rings. The task of the rotor retaining ring is, inter alia, to fix the axial end regions of rotor windings. The outside diameter of the rotor retaining rings is greater than the outside diameter of the middle part of the rotor, the so-called rotor barrel. Even at these locations, the minimum dimension of the air gap must be ensured, without the air gap dimension inside the stator, consequently in the active region of the machine, being increased. The diameter of the stator bore must therefore be increased in the axial end regions toward the end faces of the stator, in such a way that, for reasons of loss, a minimum necessary radial dimension relative to the rotor retaining ring is ensured in the region of said ring. This minimum dimension must also be maintained in order to make it possible to lift the rotor when it is being drawn along. At the same time, it is highly disadvantageous to increase the inside diameter of the stator bore abruptly in the region in question, since this is located in the still active part of the rotor. Sharp steppings of the bore diameter of the stator laminations in this region would lead to the stator laminations being subjected to axial magnetic fields and to eddy currents resulting from these.
It is known, for example, from SU 1185498 to configure the axial termination of the stator bore in such a way that the magnetic field lines end as exactly as possible. For this purpose, the stator bore is enlarged toward the end face with an increasing gradient, the secondary effect of this being that a sufficient air gap dimension in the region of the rotor retaining ring is also ensured.
It is known, furthermore, from ABB Technik 1/96, page 20 ff, for air-cooled generators to design the stator laminations toward the end face of the stator with a more or less linearly increasing inside bore diameter. This results in a conical widening of the stator bore, which, with a corresponding geometric design of the conicity, makes it possible to maintain the minimum dimension for the air gap. As compared with an abrupt increase in size, the conical widening of the stator bore affords the advantage that only a small region of each stator lamination is subjected to axial magnetic fields, with the result that the load on each individual stator lamination by induced eddy currents remains low. It is specified, moreover, in the case of air-cooled generators, to use a solid aluminum press plate connected operatively to nonmagnetic press fingers, instead of the laminated press plate known from water-cooled generators with a higher unit rating. An advantage is to be seen, here, in that the solid aluminum press plate ensures that the stator laminations are shielded effectively against axial magnetic fields in the end regions of the stator and assists in reducing eddy current losses. A disadvantage of the cited version of the so-called stair-like end stepping of the stator bore is, on the one hand, from a manufacturing point of view, that a large number of stator laminations with different bore diameters are required. Furthermore, with the increasing diameter of the stator bore, the overlap of the conductor bars of the stator is reduced. In turn, however, the conductor bars are not provided for this orientation of the magnetic field lines, and eddy currents which locally cause high thermal load are induced in the conductor bars. Particularly in the case of air-cooled generators and further-increased power output densities, these eddy current losses in the conductor bars may lead to undesirable or even inadmissible local heating and to adverse effects on the efficiencies capable of being achieved.
The invention is intended to remedy this. The object of the invention, as defined in the claims, is, by means of a novel geometry of the end region of the stator bore, to provide an improvement in the magnetic flux, to reduce eddy currents, and, consequently, lower local heating. The invention is suitable very particularly advantageously for use under the special conditions of the abovementioned air-cooled generators.
According to the invention, in an electric machine of the type initially mentioned, the end region of the stator or the stator bore is designed in axial profile with zones with a different profile of the inside diameter of the stator bore. In this case, a first zone of the stator bore has a constant clear width. For a given rotor, this clear width is predetermined by a radial air gap dimension between the rotor barrel and the laminated stator core. Particularly with regard to the air-cooled generators mentioned, which come within a power output range of up to, for example, 500 MVA, this dimension is of the order of magnitude of a mechanically necessary dimension which must be ensured for the handling of the rotor within the stator bore, for example when the rotor is being moved in and out. At the same time, this minimum dimension must be ensured even in the region of the rotor retaining rings which have a larger diameter than the rotor barrel. According to the prior art, therefore, the stator bore is designed, as mentioned above, with continuous stair-like stepping, along with the problems, likewise mentioned there, of the losses induced by radial magnetic fields in the conductor bars.
Too small a radial distance between the rotor retaining ring and the laminated stator core not only causes problems with mechanical handleability, but also an increase in the slot harmonics induced in the rotor retaining ring.
The invention is intended to specify the configuration of the stator bore end regions in such a way that, on the one hand, a minimization of the eddy current losses in the stator conductor bars is achieved, but, at the same time, it is also advantageous with regard to the further aspects.
According to the invention, a second zone with a bore diameter widening toward the end face of the stator is arranged axially outside the first zone. There follows, in the axial region of the rotor retaining ring, a third zone which, in turn, has a substantially constant bore diameter which, however, is greater than the bore diameter in the first zone. The bore diameter in the third zone is dimensioned such that, at least where conductor material is arranged, the conductor bars of the stator winding are, at least for the most part, overlapped in the circumferential direction by stator teeth. The diameter of the stator bore in the third zone is dimensioned such that at no point in the third zone is it substantially greater than an effective stator winding diameter which is defined by a radially inner boundary of the conductor material of the stator winding. As a result, the conductor bars of the stator winding in the third zone are largely shielded from radial magnetic fields which lead to losses in the stator winding.
In a preferred variant of the invention, the diameter of the stator bore in the third zone is designed in such a way that the radial dimension between the rotor retaining ring and the stator in the third zone does not appreciably exceed the air gap dimension in the first zone of the stator. This, on the one hand, ensures, the maximum overlap of the conductor bars; on the other hand, the mechanical handleability of the rotor is restricted by the minimum radial gap dimension present, and because of this an increase in the radial gap dimension in the third zone above the air gap dimension in the first zone is of no benefit. If, as stated above, the radial air gap dimension in the first zone is already near the minimum value necessary for handling reasons, the diameter of the stator bore in the third zone will preferably have to be dimensioned such that it exceeds the bore diameter in the first zone by the same amount as that by which the diameter of the rotor retaining rings exceeds the diameter of the rotor barrel.
A further design criterion for the diameters of the various zones of the stator bore is a conductor exposure coefficient QD which is defined as       Q    D    =                    D        III            -              D        I                            D        eff            -              D        I            
In this, DIII represents the diameter of the stator bore at an axial position of the third zone, DI the diameter of the stator bore in the axially inner first zone and Deff the above-defined effective stator winding diameter. The conductor exposure coefficient QD is therefore to be interpreted as the increase in diameter of the stator bore from the first zone to an axial position of the third zone, in relation to the overlap of the conductor bars in the first zone of the stator. According to the invention, this conductor exposure coefficient is selected higher than 0.5 and lower than 1.1, preferably in the range between 0.9 and 1.05. A selection of the conductor exposure coefficient near 1 is to be preferred, particularly in the case of very confined conditions of space, since this value ensures, on the one hand, a large radial gap and, on the other hand, a good overlap of the conductor bars.
As compared with the prior art having the continuous widening of the diameter of the stator bore, the design according to the invention of the end region of the stator bore affords the further advantage that a smaller number of differently manufactured stator lamination variants is required.
Furthermore, in a preferred embodiment of the invention, a fourth zone with a conical diameter profile widening toward the end face terminates the laminated stator core axially. In further conjunction with an aluminum press plate, this results in an advantageous configuration of the laminated core in terms of magnetic field lines running axially.